1. Field of Invention
This invention relates to energy conversion and more particularly to methods and apparatus for controlling power drawn from an energy converter operable to convert energy from a physical source into electrical energy.
2. Description of Related Art
Energy conversion devices such as photovoltaic arrays are commonly used to provide power to electrical loads. Often these loads are direct current (DC) loads such as batteries, for example. Recently, efficiencies in power conversion devices are giving rise to solar power systems that supply power to an alternating current (AC) load such as an AC power grid such as may be operated by a public utility company. Such power systems may employ a photovoltaic array and an interface for converting power in a form received from the photovoltaic array into a form operable to be received by the AC power grid. Such an interface may involve a DC to AC inverter.
Interfaces of the type described above often seek to cause maximum power to be provided to the AC power grid. The maximum power available to be provided to the AC power grid depends upon the conditions under which the energy conversion device is operated and in the case of a photovoltaic array, these conditions include the amount of insolation and the temperature of the array, for example. A maximum power point, or voltage at which maximum power may be extracted from the array, is a desirable point at which to operate the array and conventional systems seek to find this point. The maximum power point changes however, due to changes in insolation and due to changes in temperature of the array and thus control systems are employed to constantly seek this point.
One way of seeking the maximum power point is to periodically perturb and observe the power output of the array and then adjust the power demanded from the array accordingly to cause the voltage of the array to be as close as possible to the maximum power point. Typically, such perturb and observe methodologies involve perturbing the present power supplied to the load by a fixed amount such as 4 watts, for example and then observing the effect on power supplied by the array and the voltage measured at the array. Perturbing involves temporarily increasing the power supplied to the load by a fixed amount such as 4 watts, for example. If the change in power is negative and voltage measured at the array drops by a significant amount, too much power is being extracted from the array and the power demand on the array must be reduced, in which case the power supplied by the array is usually reduced by some fixed incremental value, such as 4 watts, for example. If the voltage does not change by a significant amount when the power is perturbed, perhaps not enough power is being extracted from the array and the present power drawn from the array must be increased in which case the power demanded from the array is usually increased by a fixed amount, such as 4 watts.
The above described perturb and observe methodology is typically conducted at the switching speed of a switching mode power supply connected to the array, e.g., 100 kHz, and results in a dithering of power drawn from the array, in fixed amounts. Where the incremental amount is 4 watts for example, as described above, there will be a constant dithering of power demanded from the array, in the amount of 4 watts about a common mode value which may be approximately equal to the maximum power output of the array. When the load is an AC power grid, the load effectively fluctuates at the line frequency of the grid, which in North America is typically 60 Hz. Consequently, the 100 kHz perturb and observe frequency of most switching mode power supplies used to supply DC loads is too fast for applications where the load is an AC power grid. Thus, the perturb and observe frequency must be decreased.
However, decreasing the perturb and observe frequency can waste power, especially when changes in insolation occur.
Changes in insolation can change the maximum power available from the array from say 200 watts to 2000 watts in a matter of seconds. This situation may occur when a cloud, for example, moves or dissipates from a position blocking sunlight shining on the array to a position in which full sun is received on the array. With 4 watt power increments, and a perturb and observe period of 50 mSec, the time to change the power drawn from the array from 200 watts to 2000 watts would be about 22 seconds. During this period the full available power is not being drawn from the array resulting in inefficient operation.